This invention relates to a thick film hybrid circuit board device formed by utilizing screen printing process and to a method of manufacturing the same. More particularly it relates to a thick film hybrid circuit board device having a high quality and a compact structure and also relates to a method of manufacturing thick film hybrid circuit boards of this character.
A thick film hybrid circuit board device of the conventional type has a construction which is a plurality of circuit forming materials laminated on a insulative base board. For example, a thick film hybrid circuit board device having a resistor element as a circuit element is manufactured by printing, e.g., screen printing a first layer as a resistor on the insulative base board and by laminating a second layer on both peripheral edge portions of the first layer as a pair of electrodes.
Resistors and electrodes are usually formed by printing circuit forming materials on a surface of the insulative base board by utilizing a screen printing process to form thick films.
The thick films printed on the base board are then fired under predetermined condition to form hardened resistors and electrodes.
In the screen printing process, a screen plate having numerous micro holes on the entire surface thereof if prepared as a printing mold. The holes are provided in an outside area of a predetermined circuit pattern formed by photomechanical processing are broken so as to be plugged.
The screen printing process is utilized as a method of forming a circuit film by squeezing a paste-like circuit forming material onto the base board through the holes provided inside the area of the circuit pattern.
The method of manufacturing the thick film hybrid circuit board device will be explained in more detail below. First,a resistive paste is screen printed on the base board to form a layer. The printed resistive paste is then fired to form a resistive layer as this first layer. Secondly, a conductive paste for forming a second layer is printed on both the upper end surfaces of the resistive layer, and the printed conductive paste is fired to form a pair of electrode layers as the second layer.
However, in this type of the conventional thick film hybrid circuit board device, stagger portions exist between the upper surface of the insulative base board and a cross portion where a peripheral edge portion of the resistive layer and that of a pair of electrode layers are mutually cross.
Due to existence of the stagger portions, a part of each of the electrode layers formed of conductive paste will spread from an internal edge portion thereof toward the other electrode layer, resulting in the formation of bleedings which move toward the other electrode layer.
For this reason, in a case where a distance between a pair of electrode layers narrows as the amount of integration of the thick film hybrid circuit board device increases the distance between the two electrode layers is substantially reduced by the bleedings which comprise the paste, so that a serious problem is encountered, that is, the resistance value of the resistive layer includes significant errors.
Moreover, in a case where the amount of paste bleedings generated from the electrode layers is large, the bleedings may mutually combine to form a bridge portion, causing a short circuit to occur between the pair of electrode layers.
Furthermore, during a trimming process for adjusting a resistance of the resistive layer, it becomes more difficult to set a starting point at which the trimming operation starts because the distance between the pair of electrode layers is extremely reduced.